This invention relates to soldering of printed circuit boards and, more particularly, to a solder paste applicator incorporating a patterned screen for applying solder to a printed circuit board for soldering surface mounted components thereon.
At present there are two primary methods of assembling electronic components to a printed circuit board (PCB). In one method, referred to as the leaded through-hole method, the components have wire leads protruding therefrom. The leads are placed through holes in the board and soldered to the circuit on the opposite side of the board. For most multi-leaded electronic components the size of the body is dependent on the number of leads that it has. The required size and spacing of the holes in the board limits the minimum distance from one lead to the next to approximately 0.100 inches.
The other more recent method involves mounting of the components directly onto the surface of the circuit board. Since there are no leaded through holes, the lead pitch can be reduced and is typically 0.050 to 0.025 inches. This allows the component body size to be reduced to as much as one-third the size required for the leaded through-hole method.
In the leaded through-hole method, the leads are soldered to the board by passing the underside of the board through a tank of molten solder. The molten solder is pushed up into the holes in the board. The board is then cooled and the solder solidifies in the holes.
Surface mounted components are soldered by applying a solder paste to the board through a fine mesh screen or thin metal stencil to control the placement of the paste on the board. When the screen is used, selected spaces in the mesh are closed to create a pattern in the open spaces which corresponds to the pattern to be placed on the board. When the stencil is used, the pattern to be created is etched directly through the metal. The screen or stencil is positioned on the board and the solder paste is spread over the screen or stencil and pushed through the openings onto the PCB by means of a squeegee. The screen or stencil is then removed, leaving the desired pattern of paste on the board. The thickness of the paste is controlled by the thickness of the screen or stencil.
In an alternate method, the solder paste is dispensed through a small diameter tube in the desired pattern directed onto the PCB. The tube may be attached to a programmable positioning system in which the paste is automatically applied to the PCB.
However, the screen print method requires that the surface of the board to which the paste is applied be free of irregularities or projections so that the screen can lay flat on the PCB. In addition, the region surrounding the perimeter of the area to be printed must be rectangular and large enough to allow room for the frame that supports the screen and to allow over-travel for the squeegee. Also, since the thickness of the deposited paste is determined by the thickness of the screen, a new screen must be made every time the thickness of the paste layer is changed. The screen must be reloaded and positioned for each surface mount board. The same disadvantages exist when using a stencil.
Although the small diameter tube solder paste dispensers can apply the paste over a range of thicknesses by using multi-orifice dispensing heads, the process is slow and inefficient for the quantities of paste required for a typical board.
It is therefore an object of the present invention to provide a solder paste application device which overcomes the disadvantages of the prior art.
It is an additional object of the present invention to provide a solder paste applicator in which the amount of paste can be regulated for each application.
It is yet another object of the present invention to provide a solder paste applicator in which varying thicknesses of paste can be applied without changing parts in the applicator.
It is still an additional object to provide a solder paste application device which allows for a reduced border area surrounding the pattern to be printed.
It is an additional object of the present invention to provide a method for applying a solder paste to a printed circuit board in a one-step operation.
These objects and others which will become apparent hereinafter are accomplished by the present invention which provides an apparatus for applying solder paste to a surface including a housing having two ends and an outer peripheral surface joining the ends, a plate removably positioned in the chamber substantially parallel to and spaced a predetermined distance from one of the ends, the plate having holes therethrough displaying a predetermined pattern, a reservoir for holding a solder paste located in the chamber and adjacent the plate and means located in the other of the ends for dispensing the solder paste through the holes and onto the surface.
The present invention also provides a method of applying solder paste to a printed circuit board including filling a reservoir housed in a chamber with a solder paste, forming a predetermined pattern of holes in a plate, positioning the plate at one end of the chamber adjacent the reservoir a predetermined distance from one end of the chamber, removing air bubbles from the solder paste, positioning the one end of the chamber on a surface of the board to be soldered, and applying pressure to the paste to force the paste to extrude through the holes and be deposited on the board to a predetermined thickness in the predetermined pattern.